Tunable waveguide devices

ABSTRACT

Methods, systems, and apparatus, including a laser including a layer having first and second regions, the first region including a void; a mirror section provided on the layer, the mirror section including a waveguide core, at least part of the waveguide core is provided over at least a portion of the void; a first grating provided on the waveguide core; a first cladding layer provided between the layer and the waveguide core and supported by the second region of the layer; a second cladding layer provided on the waveguide core; and a heat source configured to change a temperature of at least one of the waveguide core and the grating, where an optical mode propagating in the waveguide core of the mirror section does not incur substantial loss due to interaction with portions of the mirror section above and below the waveguide core.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional PatentApplication No. 62/274,377, filed Jan. 4, 2016, and U.S. ProvisionalPatent Application No. 62/379,682, filed Aug. 25, 2016 which areincorporated by reference herein.

TECHNICAL FIELD

The present disclosure is directed to tunable waveguide optical devices.In general, a heater may be used to change a characteristic of anoptical device. For example, an operating wavelength of a semiconductorlaser may be tuned by applying heat using a heater.

SUMMARY

In a general aspect, the subject matter described in this specificationcan be embodied in a laser including a layer having first and secondregions, the first region including a void; a mirror section provided onthe layer, the mirror section including a waveguide core, at least partof the waveguide core is provided over at least a portion of the void; afirst grating provided on the waveguide core; a first cladding layerprovided between the layer and the waveguide core and supported by thesecond region of the layer; a second cladding layer provided on thewaveguide core; and a heat source configured to change a temperature ofat least one of the waveguide core and the grating, where an opticalmode propagating in the waveguide core of the mirror section does notincur substantial loss due to interaction with portions of the mirrorsection above and below the waveguide core.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a cross-section view of an example tunable waveguidedevice.

FIG. 2A illustrates an example tunable section of a tunable laser.

FIGS. 2B and 2C illustrate example etch profiles.

FIGS. 3A-3C illustrate an example tunable section of a tunable laser inwhich no heater is provided on the top of the upper cladding layer.

FIGS. 4A-4B illustrate an example tunable section for a tunable laser.

FIG. 5 illustrates an example tunable section for a tunable laser.

FIGS. 6A-6E illustrates an example tunable laser.

FIG. 7 is a plot of an example temperature distribution along a tunablesection of a tunable laser.

FIGS. 8 and 9A-9C show examples of heater placement adjacent to a gainsection.

FIG. 10 illustrates an example arrangement of a tunable laser.

FIG. 11 illustrates a cross-sectional view of a tunable laser.

FIG. 12 illustrates a cross-sectional view of a tunable laser.

FIGS. 13A and 13B illustrate cross-sectional views of a heaterconsistent with the present disclosure.

FIGS. 14A-14D illustrate an example tunable laser.

FIGS. 15A-15B illustrate an example tunable laser.

FIG. 16 shows a graph illustrating a tradeoff between bandgap energy andindex contrast.

FIG. 17 shows an example tunable laser having a tapered reflector.

FIG. 18 shows an example tunable laser having a tapered gain section.

FIG. 19 illustrates a cross sectional view of an example tunable sectionthat is taken along a leg of a tunable laser.

FIG. 20 illustrates a comparison of operable regions for a wavelengthtunable laser with and without absorption reduction.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

A tunable laser enables an operating wavelength of a laser to beadjusted over a tunable wavelength range. Tunable lasers such assemiconductor laser diodes typically have a gain section and an optionalphase section provided between a pair of reflectors or mirrors (theterms “reflector” and “mirror” may be used interchangeably herein). Thegain section includes a p-n junction, and the phase section adjusts thephase of light in the laser cavity between the reflectors. A reflectormay be a grating-based reflector, which includes a waveguide having aperiodic refractive index variation corresponding to a particularwavelength of light output from the laser. For example, the reflectorshave a reflectivity characteristic that may include a series ofuniformly spaced reflection peaks, which resemble a comb. The spectraldistance between successive peaks in the comb or pitch of one reflectormay be different than the spectral distance between successive peaks ofthe other reflector. Each “comb” may be spectrally shifted by tuning thereflectors and phase sections to select a single wavelength over a widerange, such as the C-Band (1530-1565 nm) or L-Band (1565-1625 nm), whenthe reflector pitches are different and designed appropriately. Thegrating-based reflector may be a partial reflector and be partiallyreflective or a total reflector and be completely reflective or nearlycompletely reflective.

In some implementations, the grating-based reflectors may be used totune the wavelength of light output from the laser. For example, anoperating wavelength of a laser may be tuned using heaters that areprovided above and/or adjacent to the grating-based reflectors. In anexemplary tuning operation, the heaters adjust the temperature of thegrating-based reflectors, such that the entire reflection comb shifts inwavelength. When both mirrors are tuned together, the laser wavelengthtunes continuously but does not provide for much change in wavelength.When the mirrors are tuned differently with respect to each other, thelaser wavelength may hop discretely and therefore change the wavelengthin larger steps. Together, common mode and differential tuning of themirrors allows the mirror to span a large and complete range ofwavelengths. It may be desirable to thermally isolate the heat generatedby the heaters in local regions to increase the efficiency of the tuningoperation. Moreover, if multiple tunable lasers are integrated on acommon substrate in a photonic integrated circuit (PIC), for example, itmay be desirable to thermally isolate the heat generated by the heatersfor one laser from the other lasers in order to maintain the stabilityof the tunable lasers on the PIC. The present disclosure is directedtoward various laser and heater structures that provide for moreefficient thermal tuning, as well as mechanical and electricalstability. For example, an undercut region may be formed under a tunablesection (e.g., a grating-based reflector and/or a phase section) of alaser to thermally isolate the tunable section from other parts of thelaser and other optical components formed on the PIC. In someimplementations, the lasers disclosed in this disclosure may be tunableover the C (1530-1565 nm), L (1565-1625 nm) bands, extended C band, orextended L-Band.

The subject matter described in this specification can be implemented inparticular embodiments so as to realize one or more of the followingadvantages. For example, by forming an undercut portion below a tunablesection, the tunable section is thermally isolated or decoupled from thesubstrate. As a result, thermal tuning of the laser reflectors ormirrors may be more efficient. In another example, by forming heaters ona tunable laser, in which the ends of the heaters are tapered ornarrowed compared to a center portion of the heater, more heat may bedissipated at the ends of the heater compared to a heater that has auniform width. Accordingly, heat may be more uniformly dissipated by aheater having tapered ends consistent with an aspect of the presentdisclosure. In addition, the heater therefore may be made more compact,consume less power, and be more efficient.

FIG. 1 illustrates a cross-section view of an example tunable waveguidedevice 100. Waveguide device 100 may be provided on substrate 102 andmay include a lower cladding 106, core 108, and upper cladding 110. Asdiscussed in greater detail below, additional layers may be provided onupper cladding 106 as part of a laser. Undercut layer 104 having anetched region or space 120 (as used herein “undercut” and “etched” maybe used interchangeably) is formed under a lower cladding 106 in orderto thermally isolate waveguide device 100 from substrate 102. Otherdevices, not shown in FIG. 1, may be laterally disposed adjacent towaveguide device 100, and etched region 120 may thermally isolatewaveguide device 120 from those other devices as well. As disclosed ingreater detail below with references to FIGS. 2-20, the tunablewaveguide device 100 may be a tunable section, e.g., a reflector or aphase section of a semiconductor laser. In other implementations, thetunable waveguide device 100 may be a tunable section of a modulator, anoptical switch, a multiplexer, a demultiplexer, or any other suitabletunable waveguide devices that may be controlled by temperature.

The substrate 102 may include silicon, indium-phosphide (InP), or anyother suitable substrate including Group IV or Group III-V semiconductormaterials in which optical devices may be formed thereon. Substrate 102may also be doped. In one example, substrate 102 may includesilicon-doped InP. The lower-cladding layer 106 and the upper-claddinglayer 110 may be formed using materials that have a lower refractiveindex than the refractive index of the core layer 108, such that anoptical mode 114 may be confined by the lower-cladding layer 106 and theupper-cladding layer 110 to propagate in the core layer 108. Theundercut layer 104 may be formed using materials that are etched at afaster rate than the other layers, such that the etched region 120 maybe formed selectively while the other layers of waveguide device remainintact.

In general, a temperature change may induce refractive index changes inthe lower-cladding layer 106, the core layer 108, and the upper-claddinglayer 110, which changes an effective refractive index of the opticalmode 114. The change in the effective refractive index of the opticalmode 114 may be used to control an optical characteristic of the tunablewaveguide device 100. For example, if the tunable waveguide device 100is a laser, an operating wavelength of a tunable laser may be changedusing a temperature control 116, as noted above. As another example, ifthe tunable waveguide device 100 is an arm of a Mach-Zehnderinterferometer (MZI), a phase shift by the MZI arm may be changed usinga temperature control 116. A source for a temperature control 116 may bea heater formed on the upper-cladding layer 110 or an electrical sourcethat heats up the tunable waveguide device 100 by passing a currentthrough the lower-cladding layer 106 and/or the upper-cladding layer110.

The etched region 120 is formed by etching away a portion of theundercut layer 104. For example, the etched region 120 may be formed byexposing portions of the undercut layer 104 using lithography, and thenwet etching the exposed portions. The etched region 120 may be empty ormay be filled with another material having a high thermal resistance.The etched region 120 increases the thermal resistance, to reduce heatflow to the substrate 102, and therefore enhances a thermal isolation ofthe tunable waveguide device 100, as noted above.

FIG. 2A illustrates an example tunable section 200 of a tunable laser.The tunable section 200 may be a reflector or a phase section of atunable laser. In this example, the tunable section 200 is formed on asubstrate 202. The substrate 202 may be a n-doped indium phosphidesubstrate (for instance InP:S, InP:Se, InP:Si) or a semi-insulating (SI)InP substrate (e.g., InP:Fe). The tunable section 200 includes alower-cladding layer 204 formed on the substrate 202. In this example,the lower-cladding layer 204 is an n-type epitaxial layer of n-type InP.The n-type epitaxial layer 204 may be designed, patterned, and etched toform holes or openings (“undercut access openings”) 206 a-206 h. In thisexample, the tunable section 200 shows eight undercut access openings.In another example, a tunable section may have fewer or more undercutaccess openings depending on the design. The undercut access openings206 a-206 h are separated from each other by non-etched portions or“legs” 208 a-208 g. The legs 208 a-208 g may be patterned by using adielectric material as an etch mask, such as SiN or SiO₂ or acombination of dielectric layers, or a combination of semiconductorlayers, or both. The undercut access openings 206 a-206 h provide accessfor a wet etchant that etches beneath the lower cladding layer 204 tocreate an undercut/void/spacing (used interchangeably in thisdisclosure) 210. In one example, the undercut portion may be formed byetching an undercut layer 212. The undercut layer 212 may be formedusing indium gallium arsenide phosphide (“InGaAsP”) or indium galliumarsenide (“InGaAs”), which has been embedded with aluminum indiumarsenide (“AlInAs” or interchangeably “InAlAs”) or aluminum galliumindium arsenide (“AlGaInAs” or interchangeably “InAlGaAs”). Both AlInAsand AlGaInAs typically have relatively high etch rates (e.g., threetimes faster or more) compared to InGaAsP and InGaAs, such that the timerequired to form the undercut 210 can be reduced and with less opticalloss to the undercut layer 212. In addition, the resultant semiconductorprofile may be more tapered. As an example, FIG. 2B illustrates anexample etch profile 201 that includes an undercut layer 236 formedusing InGaAs. FIG. 2C illustrates an example etch profile 203 thatincludes an undercut layer formed using an InGaAs layer 238, an AlGaInAsor AlInAs layer 242, and another InGaAs layer 240. The etch profile 203is more tapered than the etch profile 201 because of less steep slope.Increased etch rates may also be achieved by introducing strain into theundercut layer 212 by a lattice mismatch. In general, an undercut voidmay be formed in the reflectors (or mirrors) and phase-tuning sectionsbut preferably not the gain section in order to keep the operatingtemperature of this section minimized (for improved performance andreliability). The thickness of an undercut layer, e.g., undercut layer212 or another undercut layer disclosed in this application, may be lessthan 2 μm or less than 1 μm.

By forming the undercut or etched region 210, the waveguide device ofthe tunable section 200 may be thermally isolated or decoupled from thesubstrate 202. As a result, thermal tuning of the laser reflectors, forexample, is more efficient.

As further shown in FIG. 2A, the tunable section 200, which may be areflector, includes a waveguide core layer 214 through which an opticalmode 216 propagates. The core layer 214 may include intrinsic ornon-intentionally doped (NID) InP or else n-type InP. The optical mode216 may extend outside the waveguide core layer 214 and into the lowercladding layer 204 and an upper cladding layer 218.

The upper cladding layer 218 may be provided on the waveguide core layer214 throughout the photonic integrated circuit and includes the laserreflectors. The upper cladding layer 218 may include InP that is dopedp-type and formed from a single epitaxial growth step. Optionally, theupper cladding layer 218 may include a layer of n-type doping having aconcentration of 10¹⁷ cm⁻³ above or otherwise spaced from the waveguidecore layer 214 to deplete holes adjacent to and especially nearwaveguide core 214 and thereby reduce optical loss of the waveguide.Alternatively, the upper cladding layer 218 may include a layer that isunintentionally doped (e.g., very low impurity levels for lower loss) orpassivated with an implant or counter doping (e.g., H or He for p-type,or O for either p or n-type). This may occur in all or part of thelayers in the reflector, but preferably the layers closest to the coreof the waveguide core layer 214 (with the highest optical overlap).

In some implementations, a p-type InGaAsP layer 220 may be provided onthe upper cladding layer 218 in a single or multiple step compositionalgrade between InGaAs and InP. In some implementations, a p+ InGaAscontact layer 222 may be formed on the p+ or p-type InGaAsP layer 220 orInP cladding layer. Graded composition layers having increasing bandgapmay also be formed going from the InGaAs contact layer 222 to theInGaAsP layer 220 below. A strip heater 224 may be formed above the p+InGaAs contact layer 222 or on any layer on top of the InP uppercladding layer wherein it is desirable to place the heaters sufficientlyfar from the vertical extent of the optical mode to result in minimaloptical loss by the optically lossy heater material (typically at least2-2.5 um from the waveguide core layer (214) to ensure minimal excessabsorption per unit length (<2-7 dB/cm) in such waveguides, althoughother thicknesses may also be employed). In some implementations, stripheaters 226 a and 226 b may be formed on the “lower mesa” LM adjacentrespective sides of the p-type upper-cladding layer 218 in addition toor instead of heater 224 provided on top mesa TM. By placing the heaters226 a and 226 b on lower mesa LM, overall stress to tunable section 200may be reduced.

In general, the height of the lower cladding layer 204 should beselected such that the optical mode 216 does not extend into the etchedregion 210, in order to minimize optical loss. For example, thethickness or distance in the lower cladding 204 between the undercutlayer 212 and the waveguide core layer 214 is preferably at least 2.5 μm(in InP or about 1.2 μm in AlGaAs) to ensure minimal excess absorptionper unit length (<1 dB/cm) in such waveguides, although otherthicknesses may also be employed.

In the example described with reference to FIG. 2A, heaters made ofmetal strips are incorporated into tunable section 200 in order tothermally tune the wavelength of light output from the laser in whichtunable section 200 is provided. As described with reference to FIGS.3-5 below, instead of using heaters, semiconductor structures of thelaser may be configured to generate heat, and thus may replace orsupplement the metal heaters described above to simplify and/or lowerthe cost of device fabrication, as well as reduce stress to the uppercladding layer 218.

FIG. 3A illustrates an example tunable section 300 of a tunable laser inwhich no heater is provided on the top of the upper cladding layer 318.Tunable section 300 may be a reflector, for example. Tunable section 300may have corresponding elements similar to those discussed above inconnection with FIG. 2A. In particular, tunable section 300 includes alower cladding layer 304, an undercut or etched region 310, an undercutlayer 312, a waveguide core layer 314, and an upper cladding layer 318.The tunable section 300 further includes undercut access openings 306a-306 h and legs 308 a-308 g. The tunable section 300 may include ap-type InGaAsP layer 320 provided on the upper cladding layer 318, and ap+ InGaAs contact layer 322 provided on the p-type InGaAsP layer 320 ordirectly on the InP cladding layer 318. As further shown in FIG. 3A, afirst electrode 330 and a second electrode 332 are provided on the lowercladding layer 304. As an example, during the operation of the tunablesection 300, a voltage (V) may be applied to the first electrode 330,and the second electrode 332 may be biased to ground. Accordingly,electrical currents 334 flow in parallel through each leg adjacent thefirst electrode 330, beneath the waveguide core layer 314, in parallelthrough each leg 308 a-g adjacent the second electrode 332, and finallythrough the second electrode 332 to ground. The currents 334 areconfined by electrical isolation trenches (“trench etch”) 336 and 338.Lower cladding layer 304 is preferably a resistive n-type epitaxiallayer. Accordingly, currents 334 may generate heat in the lower claddinglayer 304, and such heat may be dissipated toward waveguide core layer314. By adjusting the voltage applied to the first electrode 330,effective refractive index changes may be induced in the reflector, forexample, and therefore an optical signal wavelength output from thelaser may be tuned.

FIG. 3B illustrates examples of voltages and currents that may beapplied to a tunable section 301 that is similar to the tunable section300. In the example shown in FIG. 3B, the tunable section (including amirror or phase section) includes 24 legs and first and secondelectrodes that extend parallel to each other on opposite sides of thetunable section. Each electrode may be continuous so that current flowsthrough each leg in parallel. The legs are electrically ganged orconnected to one another on a first side adjacent the first electrodeand on a second side adjacent the second electrode. In this example, thelegs are spaced from one another by a 25 μm pitch and each leg is 4 μmwide. The maximum temperature change is about 100° C., and the thermalresistance is about 730° C./W. The length of the tunable section isabout 600 μm.

In some other implementations, the first and second electrodes may beconfigured, such that current flows in parallel through first and secondgroups of twelve legs each, and the first and second groups areconnected in series. FIG. 3C shows temperature distribution plot A alongthe tunable section shown in FIG. 3B in which, as noted above, thecurrent flows through all 24 legs parallel. FIG. 3C further showstemperature distribution plot B along a tunable section in which currentflows though the series connected first and second groups of twelve legseach. As further shown in FIG. 3C, plot A is more uniform along theheated section, while plot B is less uniform and peaks substantially atthe midpoint of the plot. Accordingly, choosing different leg widthsand/or spacings near center of device may minimize temperature peak ordip at center of a tunable section.

FIG. 4A illustrates another example tunable section 400 for a tunablelaser. Similar to the elements as described in reference to FIG. 2A, thetunable section 400 includes a lower cladding layer 404, an undercut410, an undercut layer 412, a waveguide core layer 414, and an uppercladding layer 418. The tunable section 400 further includes undercutaccess openings 406 a-406 h and legs 408 a-408 g. The tunable section400 may include a p-type InGaAsP layer 420 provided on the uppercladding layer 418, and a p+ InGaAs contact layer 422 provided on thep-type InGaAsP layer 420 or directly on the InP layer 418. In thisexample, a first electrode and a second electrode are segmented intofirst electrode sections 430 a-430 g and second electrode sections 432a-432 e. Each of the first electrode sections 430 a-430 e iselectrically isolated from one another. Similarly, each of the secondelectrode sections 432 a-432 e is electrically isolated from each other.As a result, when a voltage is applied to one of the electrode sections,as further shown in FIG. 4A, current 434 flows through a correspondingone of the legs (e.g., 408 e) adjacent to the first electrode section430 a-430 m and through the lower cladding layer 404 (e.g., ann-epitaxial layer) to one of the legs (e.g., 408 a) adjacent one of thesecond electrode sections 432 a-432 m. The current 434 next flows backthrough an adjacent leg (e.g., 408 b) connected to the second electrodesection and back to another one of the first electrode sections via thelower cladding layer 404. The current 434, therefore, may flow in aserpentine manner, as indicated by the arrows shown in FIG. 4A extendingbetween the first and second electrode sections 430 a-430 m and 4302-432m, until the current sinks to ground.

Referring to FIG. 4B, which shows a top view of the tunable section 400,the leg pitch may be 5-100 μm, the leg or arm width may be 1-30 μm, andleg length can be 2-50 μm, for example.

FIG. 5 illustrates another example tunable section 500, including amirror or phase section of a tunable laser. Similar to the elements asdescribed in reference to FIG. 2A, the tunable section 500 includes alower cladding layer 504, an undercut 510, an undercut layer 512, awaveguide core layer 514, an upper cladding layer 518, a p-type InGaAsPlayer 520 provided on the upper cladding layer 518, and a p+ InGaAscontact layer 522 provided on the p-type InGaAsP layer 520. Although theupper cladding layer 518 and the contact layer 522 are described as bothbeing p-type, in some implementations, both may be n-type. In eithercase, the doped material may be relatively narrow and thin to besignificantly resistive, such that simply running a current from one endof upper cladding layer 518 or contact layer 522 to the other wouldrender voltage requirements impractical. Therefore, these layers, whichmay constitute a relatively long resistor, may be segmented intosections, which are driven in parallel (e.g. 10-20 sections run inparallel) to provide a significantly lower resistance. Current may alsoflow through the waveguide, including lower cladding 504, core 514, andupper cladding 518, in parallel with the current flow.

Moreover, in another example, the contact layer 522 may be replaced byanother semiconductor material, such as amorphous silicon or polysiliconthat is provided above upper cladding 518 and whose resistance may beadjusted by doping to achieve the appropriate resistance for heating anddriving contact layer 522 with a desired power supply.

Preferably, P-type III-V material for the contact layer 522 is doped tohave a concentration of 10¹⁸ to 10²⁰ cm⁻³ to provide suitableresistance, and the thickness may be in a range of 500-5000 Angstromsfor processing convenience. P-type silicon or n-type III-V material (orsilicon) can be doped to 10¹⁷ to 10²⁰ cm⁻³ and layer thickness may bewithin a range of 500-5000 Angstroms. The appropriate doping level andnumber of parallel electrodes used may be selected based on the lengthof the section to be heated, the resistance requirements of the circuit,thickness of the heater, mobility, and material limitations (e.g.,doping concentration limit). As an example, the electrode may be inseries or broken into up to 30 parallel sections. In someimplementations, the electrodes may be connected by air bridges, asdiscussed in greater detail below with respect to FIG. 9A.

As further shown in FIG. 5, alternating first and second contacts, suchas metal contacts 540 a-540 d, may be provided on the heavily dopedsemiconductor contact layer 522. The first contacts (e.g., 540 b and 540d) may be biased to a desired voltage and the second contacts (e.g., 540a and 540 c) may be biased to ground. The voltage may be selected suchthat a current flows away from each of the first contacts 540 b and 540d and toward adjacent second (ground) contacts 540 a and 540 c incontact layer 522. Accordingly, a desired level of heat may be providedso that the tunable section 500 has a desired temperature that yields aselected wavelength of light output from the laser. Changing the voltageapplied to the first contacts 540 b and 540 d, and thus the amount ofcurrent, as noted above, may result in corresponding changes inwavelength.

In the above examples, heaters may be provided either adjacent the gainor phase section of the laser. In addition, the gain and phase sectionsmay be either deep etched (i.e., etched through the core layer) orshallow etched (i.e., does not etch through the core layer) as a “ridge”waveguide. In some implementations, the gain section is shallow etchedbecause it is biased by current injection (i.e., current flows down theridge). As a result, the etch does not go through the p-n or p-i-njunction, for improved reliability. The other sections may be shallowetched to provide less loss and back-reflection between sections withinthe laser, or may be deep etched for tighter optical and thermalconfinement. In addition, the mirror waveguide may be flared to a widthof 2-8 μm to enable lower resistance, more manufacturable heaters viacontacts and heaters, as discussed in greater detail below with respectto FIGS. 19A and 19B

The heater may be provided adjacent the gain section, either as a metalor semiconductor heater in a manner similar to that described above. Byvarying the temperature of the gain section, the phase of light of thelaser cavity may be changed. Accordingly, by varying the phase byapplication of an appropriate temperature to the gain section, aseparate phase section in the laser may be omitted, thereby simplifyingdevice design and making the device more compact.

Regardless of the whether the gain section is tuned, the undercutpreferably does not extend beneath the gain section, because the gainsection is preferably thermally coupled to the substrate to ensure thelowest operating temperature for minimal Auger recombination loss,carrier leakage loss and for improved reliability. In this manner, thegain section, which may generate a significant amount of heat, can beadequately cooled by a heat sink, for example, that draws heat from thegain section through the substrate. On the other hand, the mirrorsections and separate phase section(s) are typically passive elementstuned by heaters with relatively higher thermal resistance. Accordingly,the mirror and phase sections are preferably thermally decoupled fromthe substrate by the undercut layer in order to provide adequatewavelength tuning.

Consistent with the present disclosure, in some implementations, thereflector section and/or the phase section of a tunable laser mayincorporate features that yield thermal uniformity in these sections.These features will next be described with reference to FIGS. 6A-6E and7. In general, thermal uniformity is important for performance andreliability for thermally tuned lasers. Non-uniform thermal gradientsalong a reflector can cause thermal hot spots that degrade thereliability of the heaters. Moreover, non-uniform thermal gradients atthe edge of mirrors can degrade the reflectivity spectrum leading toreduced performance, especially less stable laser wavelength control.

Thermal uniformity has predominant impact on the mirror and laserperformance, whereas periodic thermal variations along the mirrornegligibly degrade mirror reflectivity. Referring to FIG. 6A as anexample, FIG. 6A illustrates a top view of a reflector 606. Thereflector 606 includes n undercut access openings per side 612 a-612 n,k legs per side 610 a-610 k, a heater 614, and a waveguide 616, where nand k are positive integer numbers. m groups of gratings 618 a-618 m areformed on the waveguide 616, where the gratings have a designed gratingburst pitch representing a fixed separation between adjacent gratinggroup centers, and where m is a positive integer number. For example,the grating burst pitch may range from 65 μm to 75 μm to enable fullC-band tunability.

In some implementations, a pitch between adjacent legs may be designedto match the grating burst pitch. FIG. 6B shows a thermal profile 603(where larger refractive index on the y-axis represents hottertemperature) of a reflector section (e.g., reflector 606) when the pitchbetween adjacent legs d1 is designed to match the grating burst pitch d2by aligning the legs with the grating bursts. As shown in thermalprofile 603, the temperature at locations along the waveguide adjacentthe etch holes (e.g., the undercut access openings 612 a-612 n) may begreater (“hot”) than the temperature at locations adjacent the legs(e.g., legs 610 a-610 k) (“cold”). In some implementations, a pitchbetween adjacent legs may be designed to mismatch the grating burstpitch. FIG. 6C shows a thermal profile 605 of a reflector section (e.g.,reflector 606) when the pitch between adjacent legs d1 is designed tomismatch the grating burst pitch d2 by not aligning the legs with thegrating bursts or any integer multiple or integer quotient thereof. FIG.6D shows an example simulation 607 of reflectivity frequency responsesfor a reflector between the matched (solid line) and mismatched (dashed)cases. In this case, a temperature difference between the undercutaccess opening and the leg is 20° C. FIG. 6D shows negligible differencein mirror reflectivity spectrum between the two cases. For example, thesupport leg dip placement relative to grating burst location has verylittle impact on the mirror reflectivity peak and thefull-width-half-maximum. Thus, it would be advantageous to design areflector section having a uniform thermal profile at the edge of themirror, instead of trying to maintain a thermal periodicity variationthat aligns with grating bursts within the reflector section. Forexample, it may be desirable to achieve a temperature profile where thepeak temperature is less than °10 C from the average temperature.Preferably, the peak temperature is less than °5C from the averagetemperature.

Moreover, if the leg support design to the grating burst locations isconstrained to grating burst pitch, it is not possible to optimize themirror thermal and structural profile as effectively. By decoupling thegrating burst pitch and the pitch of the legs, more flexible designs ofthe leg and undercut access opening placements is enabled. In someimplementations, support leg pitch may be as much as a factor of twoless than the grating burst pitch, when factoring in desired thermalresistance and structural support. For example, grating burst pitch mayrange from 65 to 75 μm to realize C-Band tunability. The pitch of legsmay be less than 75 μm and preferably less than 50 μm. Nominal legsalong optical axis (e.g., leg width as shown in FIG. 6A) may range inwidth from 2 to 12 μm, and preferably 3 to 7 μm.

At the mirror edges, it is advantageous to have the freedom to vary theouter one, two, or more window openings to peak the thermal resistance,compensate heat flow to the non-undercut ends of the waveguide, and tooptimize the stepped ΔT profile. FIG. 6E illustrates a top view of anexample tunable laser 600. The tunable laser 600 includes a gain section602, a phase section 604, a first reflector section 606, and a secondreflector section 608. The first reflector section 606 and the secondreflector section 608 may be implemented using any tunable section asdescribed in reference to FIG. 1 to FIG. 5 above. In someimplementations, the width and/or spacing of the legs may be selected toprovide a substantially uniform temperature distribution along thelength of the tunable mirror section or purposely tune the temperatureprofile along the length of the laser. For example, the first reflectorsection 606 may include legs 610 a, 610 b, 610 c, 610 d, and 610 e. Thespacing between the legs (e.g., 610 a, 610 b, and 610 c) closest to theends of a tunable laser section may be greater than the spacing betweenremaining legs (e.g., 610 d and 610 e) of the tunable laser section.

Additionally, the distance along the waveguide required to transitionbetween the uniformly hot portion of the heated section to the nextportion of the waveguide, such as the gain section 602 or the phasesection 604, is preferably minimized in many cases in order to minimizean effective index change between a heated section (e.g., the secondreflector section 608) and a gain section (e.g., the gain section 602).As a result, the heat required for a given amount of tuning can beminimized or reduced, as well as the distance between reflectors in thelaser cavity. As an example, to minimize the length of the thermalgradient or transition region, the spacing between the first ˜100 μm, orfirst through fourth pairs of legs closest to the ends of the tunablelaser section, may be greater than the spacing between remaining legs ofthe tunable laser section. Such spacings may result in a thermalgradient similar to that shown in FIG. 7 between hot and cold locations,in which the gradient portion of the plot is relatively sharp and shortin length and shows a temperature change of more than 90 degrees C. over50 μm as opposed to over 100 μm in a laser consistent with the presentdisclosure. Another method to minimize the thermal gradient is to usenarrower legs in the first 100 μm or first four leg pairs at the ends ofthe heated sections. The mirror gratings, for example, may not beaccurately tuned if formed in material having a net sloped thermalgradient. Thus, a sharp, but short, thermal gradient, can reduce thecavity length, resulting in a more compact laser design. As an example,the width of the opening 612 a may be designed to be between 30 to 45μm, the width of the opening 612 b may be designed to be between 10 to20 μm, when the width of the opening 612 c is designed to be 10 μm, andall widths scaled accordingly as the width of opening 612 c is variedfrom 10 um.

Moreover, a sharp gradient can minimize temperature increases in thegain region resulting from the heated mirror and phase sections. Thesharp gradient also minimizes the heat required to tune the waveguidemirror. Generally, heating the gain section causes reduced reliabilityand reduced performance due to hot spots and reduced quantum efficiency.Accordingly, the gain section is typically not heated over a widetemperature range, but may be tuned over a small range for phasecontrol.

Returning to FIG. 6E, in some implementations, a heater 614 (e.g., theheater 224 from FIG. 2A) may be tapered toward heater ends 614-1 and614-2, such that a middle section (614-3) of each heater is wider thanthe end portions. For example, the heater may be tapered or stepped overa length of 10 to 150 μm. As a result, more heat may be dissipated atthe ends so that the heater may have a more uniform temperature over alarger portion of the heater. Also, the heater may be made more compactto enable the laser cavity to be shorter or else the gain section may bemade longer for the same cavity length. The tapered sections of theheater may be linear (as shown), sub-linear or super-linear. Multipletapers may be employed in the heater geometry, including those over themirror portion as shown, as well as additional tapers to allow largerlandings for contact vias beyond the ends of the mirrors.

Excessive thermal resistance in the mirror section (as measured perlength of the heated mirror section) can destabilize the wavelengthtuning of the laser. For example, stable wavelength tuning has beenobserved for ratios of the thermal resistance (C.°/W) to heated mirrorlength (μm) that is less than about 1.3 C.°/W/μm. However, removing thecontact layers allows the design to exceed this empirical limit byreducing optical absorption in those layers, and is described in moredetail with reference to FIGS. 9A-9C.

FIGS. 8 and 9A-9C show examples of heater placement adjacent to a gainsection 800, which serves as a phase tuning element. In FIG. 8, a heater802, which may include tungsten, for example, is provided in or above aplanarization dielectric 804 (e.g., a layer of bisbenzocyclobutene(“BCB”)), and an electrode 806 that supplies current to the gain sectionoverlies the BCB layer 804 and contacts gain section 800, which mayinclude a lower cladding layer 808, a waveguide core layer 810, an uppercladding layer 812, and a contact layer 814. In some implementations, ifreactive materials are employed or if necessary for adhesion, the heater802 may be encapsulated in dielectric layers 816 and 818 (e.g., SiN,SiOxNy, SiOx or combinations thereof).

FIG. 9A shows an example arrangement a gain section 900, where theplanarization material (e.g., BCB) is removed or omitted to reducestress and/or stress changes over life of a laser. In the example shownin FIG. 9A, a heater 902, including platinum and/or tungsten is providedon a side of the gain section 900 opposite to where the current carryingelectrodes 922 and 924 (e.g., leads or wires) are provided. The currentcarrying electrodes 922 and 924 are shorted using an isolated metalstrip 906 that is formed using a metallization step that is differentfrom the metallization step that forms the electrodes 916, 922, and 924.The gain section 900 may include a lower cladding layer 908, a waveguidecore layer 910, an upper cladding layer 912, a contact layer 914, and anelectrode 916. In FIG. 9A, the planarization material (e.g., BCB) isremoved or omitted using a metal air-bridge process. A conventionalmetal air-bridge process uses photoresist or other dissolvable organicmaterial to define the bottom of the metal path between at least twodifferent landing or contact areas so that a combination of metalevaporation, sputtering or electroplating initiates and builds up themetal bridge to a desired thickness and in the desired location. Afterdissolution of the organic material, the metal connection or bridgebetween the two or more contacts is free-standing over a range oftopography including trenches, flat surfaces and sometimes higherfeatures. As noted above, the undercut section does not extend beneaththe gain section. Further, BCB may be removed along the heater and phasesections, but typically not adjacent to the gain section.

FIG. 9B illustrates a simplified plan view of an example laser 901including reflector 932 and 934, a phase section 936, and a gain section938, whereby the planarization material (e.g., BCB) is removed orcleared adjacent the gain section 938. FIG. 9C illustrates an example ofa laser 903 similar to that shown in FIG. 9B, including reflector 942and 944, a phase section 946, and a gain section 948, but with theplanarization material (BCB) adjacent the gain section 948.

If a phase section is included, the undercut portion, as shown in FIG.2A, may or may not extend beneath such phase section depending on thelength of the phase section and the thermal budget of the laser.Accordingly, thermal decoupling of the phase tuning sections from thesubstrate may or may not be required.

Preferably, the gain section, is thermally isolated from other heatedsections, such as the mirrors. FIG. 10 illustrates an examplearrangement 1000 of a tunable laser including a heated reflector 1002and a gain section 1004. As shown in FIG. 10, a layer 1006 including aheat sink metal, for example, may be provided between the reflector 1002and the gain section 1004 that extends to the substrate as a “coldfinger.” Alternatively, other materials may be used as a “cold finger”.Further, other devices may be used to thermally isolate the gainsection, such as a heat sink. The cold finger may be made of gold andmay have a width in a range of 1-50 μm and a thickness in a range of1-10 μm. Preferably, the dimensions of the gold finger are such that thegold finger does not interact with an optical mode propagating in thewaveguide.

FIG. 11 illustrates a cross-sectional view of a tunable laser 1100. Thetunable laser 1100 includes a lower cladding layer 1102, a waveguidecore layer 1104, an upper cladding layer 1106, a contact layer 1108, anda heater 1110. In this example, the heater 1110 may be provided on thewaveguide upper cladding 1106, including InP, for example. The heater1110 may include one or more of the following materials: Ta, WN_(x), W,TaN, Cu, Al, WSi, WNSi. W or WN_(x) or WNSi is potentially preferred forInP as it has a nearly matched coefficient of thermal expansion (CTE) tominimize stress on the heater 1110. In some implementations, if theheater material is reactive, e.g., the heater 1110 is made of tungsten(W), the heater 1110 may be fabricated with a vertical or positive sideslopes, and may be fully encapsulated in a dielectric 1112, such as SiN,SiON, or SiO₂ as all or parts of the encapsulation layer 1112. Theseencapsulation layers 1112 are preferably sufficiently thin to not addadditional stress, but sufficiently thick to provide environmentalsealing. For example, the minimum thickness of the encapsulation layers1112 may be 0.5 μm. Typically, the heater 1110 may constitute a strip ofmetal, and the coefficient of thermal expansion (CTE) of the heater 1110and the encapsulating dielectric 1112 is selected to reduce stress.Although not shown in FIG. 11, in some implementations, an undercut maybe formed under the lower cladding layer 1102 as described in referenceto FIG. 2A.

FIG. 12 illustrates a cross-sectional view of a tunable laser 1200. Thetunable laser 1200 includes a lower cladding layer 1202, a waveguidecore layer 1204, an upper cladding layer 1206, a contact layer 1208, aheater 1210, and optionally a dielectric 1212. As shown in FIG. 12, atop portion of the heater 1210 is exposed such that a “landed via” 1214can be provided that contacts the heater 1210. In some implementations,the landed via 1214 has positive slide slopes 1214-1 and 1214-2 toensure that a metal sealed contact 1214-3 does not run over the sides ofthe waveguide. Although not shown in FIG. 12, in some implementations,an undercut may be formed under the lower cladding layer 1202 asdescribed in reference to FIG. 2A.

In some implementations, the heater may include a stacked structureincluding alternating layers of different metals. Referring to FIG. 13aas an example, a stacked heater 1300 including a layer of platinum 1302provided between two layers of titanium 1304 and 1306 has been found tohave better adhesion than other heater materials. In another exampleshown in FIG. 13b , a stacked heater 1310 may include alternating first(1312, 1316, 1320) and second (1314 and 1380) layers, wherein the firstlayers (1312, 1316, 1320) include a metal, for example, having acoefficient of thermal expansion (“CTE”) less than that of the substrateand the second layers (1314 and 1380) include a metal, for example,having a CTE greater than that of the substrate or vice versa. As aresult, the stacked heater 1310 may be stress-balanced, such that theoverall stress is reduced to prevent delamination. Preferably,differences in CTE are not be greater than 5 ppm/C—between dielectricand semiconductor or between heater metal and semiconductor. The stackedheater 1310 may be used in any one of the tunable sections described inthis disclosure, or any other thermal-controlled optical devices.

A reflector may be controlled to change the operating wavelength of thelaser over a wide range. In order to insure such tunability, a reflectoris preferably designed to have a reflection spectrum that is shaped toprovide maximum reflectance at a desired wavelength or comb ofwavelengths. Any distortion relative to such design can degrade laserparameters such as the side mode suppression ratio (a ratio of theamplitude of the main mode to the largest side mode), laser thresholdand optical linewidth. Such distortions can be minimized by reducingoptical absorption in the reflector.

Because the optical field intensity is high inside a laser cavity, arelatively small amount of optical absorption in a reflector can lead tosignificant distortions in the reflection spectra. For example, opticalabsorption can induce distortions by non-uniformly changing the materialrefractive index along the length of a reflector waveguide section. Inparticular, such refractive index changes may result from non-uniformthermal heating in the reflector waveguide due to the optical energyabsorbed by the material. In addition, refractive index changes can becaused by photo-carrier generation due to optical absorption andsubsequent carrier accumulation in low bandgap material layers of thereflector. In the presence of high electric fields, which may be foundin the mirror, non-linear phenomena, such as two-photon absorption(“TPA”), may cause further absorption. Moreover, hot-carrieraccumulation in passive semiconductor material layers of the mirror canenhance optical absorption due to a greater interaction cross-sectionfor hot carriers.

Consistent with the present disclosure, in some implementations, thereflector section of a tunable laser may incorporate features thatreduce absorption. These features will next be described with referenceto FIGS. 14A-14D, 15A, 15B, and 16-18.

In general, a wavelength tunable laser (WTL) requires doping andcontacting schemes in the gain section of the laser. This requirement,along with practical integration schemes for tunable mirrors and tunablephase sections, means that some or all doping and contacting layers areoften present in the mirror and/or phase tuning sections of the WTL. Ifoptical absorption is high, self-heating would occur, which may affectthe grating stability, especially for mirrors with high thermalresistance values (>200° C./W, and especially >500° C./W undercutsections for example). An important consideration for the design of astable device is to have sufficiently low optical absorption in theundercut sections, to ensure that self-heating does not occur withinthese sections in the cavity. In the absence of sufficiently lowabsorption in the undercut sections, the tuning characteristics andstability of the laser are significantly affected. As an example, FIG.20 shows a comparison of operable regions for a WTL with and withoutabsorption reduction in the undercut layers. The horizontal axesrepresent the power that is applied to the two tunable mirror heaters.The vertical axis represents the operating wavelength for given appliedpowers. The flat region, e.g., region 2002 or 2004, is a usable regionfor a WTL. Outside of these flat island regions, the WTL would exhibitunusable multimode behavior. By reducing the absorption in the mirrorsections, the usable region 2004 is notably larger than the usableregion 2002, which enhances device stability and reduces the controlrequirements placed on the laser.

Moreover, while the undercut regions inherently do not absorb light, therest of the PIC may contain a non-undercut layer, and the design shouldbe such that absorption over the gain section and for the propagationlength of the chip is minimized to less than 1 dB/cm or less than 3 dBfor most practical circuits. Additionally, the contact layer especiallyin the undercut elements should be minimized to less than 1 dB/cm toavoid localized heating and distortion of the mirror reflectanceproperties. A thermal gradient of more than 20° C. owing to absorptionby the contact can noticeably degrade the reflector characteristics, andthe amount of absorption required to produce such a gradient is afunction of the laser design as well as the thermal resistance perlength of the undercut reflector. Failure to adequately eliminatecontact absorption and reflector distortion can result in laser tuningcharacteristics that have hysteresis and therefore poor control of thelaser wavelength. Elimination of the contact layer by design or byprocess over the reflectors is the most robust way to ensure thatadverse effects of absorption-induced distortion are avoided. It is alsoadvantageous to minimize loss and convenient to remove the contact layerwherever possible from routing waveguides, couplers and separatephase-tuning sections if such an elements are employed in the laser orin the circuit.

As discussed below, there are various ways for achieving sufficientlylow-absorption in mirrors and phase sections that are undercut. Forpurposes of this invention, we define a “low loss undercut section” tohave a total modal loss less than 2.5 dB/cm, less than 5 dB/cm, or lessthan 7 dB/cm depending on the design requirements. Accordingly, a WTLthat has a low loss undercut section is a laser where the reflectorsection with a modal loss in that section that is less than 2.5 dB/cm, 5dB/cm, or 7 dB/cm. If the WTL has an undercut phase section, it may alsobe desirable to have that undercut section be a low loss undercutsection. In some implementations, this may be achieved by removingabsorbing contact layers from undercut sections. Absorbing layers forpurposes of this patent are layers where bandgap wavelength is longerthan the operating wavelength of laser. A deleterious device layer is acontact layer or other layer within the device outside the waveguidecore which absorbs the optical power from a propagating signal to causea modal loss that is higher than a desirable loss, e.g., 2.5 dB/cm, 5dB/cm, or 7 dB/cm. The deleterious device layer may be a p-type or ann-type layer. FIG. 14A shows an example tunable laser 1400 provided onsubstrate 1402, and FIG. 14B shows a side view of laser 1400 taken alonga direction indicated by an arrow 1402. Both figures may be referred toin the following text. In this example, the tunable laser 1400 includesan n-type InP substrate 1402. Other substrate types may be suitable. Anundercut layer 1420 is formed on the substrate 1402, and a lowercladding layer 1422 is formed on the undercut layer 1420. A waveguidecore layer 1404 is formed on the lower cladding layer 1422. An uppercladding layer 1406 including a p-type layer 1407, an n-type layer 1408,and a p-type layer 1409 is provided on the waveguide core layer 1404.The tunable laser 1400 further includes a p+ contact layer 1410, asdescribed in greater detail below, that may be provided on selectedportions of the p-type cladding layer. The tunable laser 1400 includes afirst reflector section 1412, a phase section 1414, a gain section 1416,and a second reflector section 1418. Other layers or structures, such asa dielectric encapsulation and a metal strip heater for thermal heating,may be included in the tunable laser 1400 but are not shown for ease ofexplanation.

The p+ contact layer 1410, which may include Indium Gallium Arsenide(InGaAs) or another narrow bandgap material, for example, may beprovided or deposited on the gain section 1416 of the tunable laser1400, but not on the reflector sections 1412 and 1418 and phase section1414. In one example, the p+ contact layer 1410 may be first depositedon and is then etched over reflection sections 1412 and 1418, as well asthe phase section 1414, to expose portions of the upper cladding 1409corresponding to these sections.

The p+ contact layer 1410 typically includes a narrow bandgap material.The high doping concentration and low bandgap of the p+ contact layer1410 renders the layer absorptive. Selectively removing the p+ contactlayer 1410 reduces absorption and loss in the reflection sections 1412and 1418, as well as in the passive routing and coupler sections notshown in FIG. 14A.

As further shown in FIG. 14A, an additional n-type layer 1408,including, for example, a layer of n-type InP, may be provided above thewaveguide core layer 1404. N-type layer 1408, as well as other n-typelayers disclosed herein may be doped with silicon, although other n-typedopants may be used. The presence of this n-type layer 1408 furtherreduces loss or absorption in the reflection sections 1412 and 1418 andthe phase section 1414. In addition, referring to FIG. 14B, similar tothe descriptions in reference to FIG. 2A, undercut regions 1424 and 1426may be formed under the reflection sections 1412 and 1418. In someimplementations, the undercut region 1424 may also extend beneath phasesection 1414.

In some implementations, one or more absorbing contact layers may beformed in the reflector section, where the absorbing contact layers areremote from the propagating mode, i.e., with sufficiently low modaloverlap to reduce absorption to achieve the desirable modal loss (e.g.<2-7 dB/cm). In some implementations, the reflector and/or the phasesection may be orthogonal or skew to the gain section to produce acompact laser, laser array, or PIC design.

FIG. 14C shows a cross section of the tunable laser 1400 taken throughthe gain section 1416 that is transverse to the propagation direction oflight in the laser. FIG. 14C further shows a heater 1430 for thermaltuning. As shown in FIG. 14C and as described in reference to FIG. 11,in some implementations, a dielectric 1432 may be provided thatencapsulates the upper cladding layer 1406 as well as the heater 1430.The dielectric 1432 may be an oxide.

FIG. 14D shows a cross section of laser 1400 taken through a reflectorsection such as the reflector 1412 (or the reflector 1418). As shown inFIG. 14D, the reflector 1412 lacks the p+ contact layer 1410, butincludes an n-type doped layer 1408, both of which contribute to reducedabsorption in the reflector 1412.

FIG. 15A shows an example tunable laser 1500 provided on substrate 1502,and FIG. 15B shows a side view of laser 1500 taken along a directionindicated by an arrow 1502. The tunable laser 1500 includes an n-typeInP substrate 1502. An undercut layer 1520 is formed on the substrate1502, and a lower cladding layer 1522 is formed on the undercut layer1520. A waveguide core layer 1504 is formed on the lower cladding layer1522. An upper cladding layer 1506 including a p-type layer 1507, ann-type layer 1508, and a p-type layer 1509 is provided on the waveguidecore layer 1504. The tunable laser 1500 further includes a p+ contactlayer 1510 that may be provided on selected portions of the p-typecladding layer. Different from the tunable laser 1400 as described inreference to FIGS. 14A and 14B, the tunable laser 1500 includes a firstreflector section 1512, a gain section 1516, and a second reflectorsection 1518. In this example, the phase section is integrated with thegain section 1516. Referring to FIG. 15B, similar to the descriptions inreference to FIG. 2A, undercut regions 1524 and 1526 may be formed underthe reflection sections 1512 and 1518.

In some implementations, a reflector (e.g., reflector 1412) may includehigh bandgap materials in the waveguide core layer (e.g., high bandgapmaterials 1450 and 1452 in the waveguide core layer 1404), which mayfurther reduce optical absorption but preserve a sufficient indexcontrast to maintain substantial confinement of light to the waveguidecore. High bandgap materials may include InGaAsP or AlInGaAs (orinterchangeably, InAlGaAs), for example. Generally, there is a tradeoffbetween increased bandgap energy and index contrast between the core andthe cladding. FIG. 16 shows a graph 1600 illustrating this tradeoff. Thegraph 1600 includes a trace 1602 showing that the scattering andcoupling loss increases as material bandgap wavelength decreases. Thegraph 1600 further includes a trace 1604 showing that the materialbandgap absorption increases as material bandgap wavelength increases.An appropriate combination of bandgap and index contrast may be selectedto provide a desired index contrast and sufficiently high bandgap thatreduces absorption.

In accordance with a further aspect of the present disclosure, in someimplementations, a reflector or a phase section may be laterally taperedor flared, as shown in FIG. 17. FIG. 17 illustrates an example tunablelaser 1700 that includes a first reflector 1702, a phase section 1704, afirst taper section 1706, a gain section 1708, a second taper section1710, and a second reflector 1712. Waveguide 1700 has flared sections,as further shown in FIG. 17. In particular, first taper section 1706between gain section 1708 and phase section 1704 (reflector 1702) has awidth w′ extending in a direction D1 transverse to direction D2 of lightpropagation in the waveguide 1700, and such width w′ narrows ordecreases in direction D2 toward gain section 1708. In addition, secondtaper section 1710 has a width w″ that increases in direction D2 fromgain section 1708 to second reflector 1712.

By flaring the waveguide from the gain section 1708 to the phase section1704 or to the second reflector 1712, optical confinement is reduced, aswell as the optical field intensity in the phase section 1704 and thereflectors 1702 and 1712. As a result, non-linear effects, such astwo-photon absorption, may be reduced. Wider heater elements and viasmay be integrated on wider mirror and phase-tuning waveguides in orderto optimize the design for required drive voltage, heater currentdensity (for reliability), and process capability of heater and viadimensions.

In accordance with a further aspect of the present disclosure, a gainsection or gain sections of a tunable laser may be laterally tapered orflared to be wider than other sections in the laser. FIG. 18 shows anexample tunable laser 1800 that includes a first reflector 1802, a phasesection 1804, a tapered gain section 1806, and a second reflector 1808.Gain section 1806 is flared. In particular, as shown FIG. 18, gainsection 1806 has a first portion 1806-1 having a width w′ in a directionD1 transverse to a direction D2 of light propagation in gain section.Width w′ increases in a direction D2 from phase section 1804 (or firstreflector 1802) to second reflector 1808. In addition, gain section 1806has a second portion 1806-2 with a width w″ extending in direction D1.Section gain portion 1806-2 narrows or decreases in direction D2 fromgain section 1806 to section reflector 1808.

By flaring the gain section 1806, a lower thermal resistance may beachieved, which improves performance of the gain section 1806 (i.e.,less Auger recombination), especially when a large current is applied tothe gain section and/or the substrate has a high temperature. Improvedperformance may also be observed in the blue (higher frequency) end ofthe lasing spectrum. Such gain section performance improvement may bemeasured through laser threshold maximum output power and linewidthacross a wavelength band of interest e.g. C-band (1530-1565 nm).

FIG. 19 illustrates a cross sectional view of an example tunable section1900 (mirror or phase) that is taken along a leg (e.g., leg 208 a inFIG. 2A) of a tunable laser. The tunable section 1900 includes a lightlydoped p-type portion 1902 provided under a portion of the lower claddinglayer 1904 (e.g., an n-type epitaxial layer) and adjacent to an undercut1906 to provide electrical isolation with minimum impact to topographythat may otherwise compromise contacts, vias, and heaters. This p-typematerial 1902 replaces the more conductive n-type material to facilitateelectrical isolation of the lower cladding layer 1904 in one portion ofthe circuit from another, which can be advantageous if the n-type(ground) portions of a PIC including tunable lasers, variable opticalattenuators (VOAs), photodiodes, phase adjusters, or other elements arebiased differently (multiple grounds on PIC), and failure to adequatelyisolate such n-type portions can result in excessive ground currents.

As further shown in FIG. 19, a dielectric layer 1908 may be providedabove the lower cladding layer 1904 and a p-type upper cladding 1920. Inaddition, a heater 1910 may be provided on the dielectric 1908, and anencapsulating layer 1912 (or planarization layer), which also mayinclude a dielectric material, may be provided over the entire device1900. Further, an opening or via 1932 may be provided in theencapsulating layer 1912 over the heater 1910. A metal or otherconductive material 1914 may then be provided in the via 1932 to provideelectrical contact to the heater 1910. In some implementations, thecontact via 1932 is provided at an end of the heater 1910.

The laser shown in FIG. 19 has a shallow ridge waveguide having a core1922, although it is understood that the waveguide may be deep etchedthrough the core.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification. It is intended that thespecification and examples be considered as exemplary only.

While this document may describe many specifics, these should not beconstrued as limitations on the scope of an invention that is claimed orof what may be claimed, but rather as descriptions of features specificto particular embodiments. Certain features that are described in thisdocument in the context of separate embodiments can also be implementedin combination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or a variation of a sub-combination. Similarly, whileoperations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results.

Only a few examples and implementations are disclosed. Variations,modifications, and enhancements to the described examples andimplementations and other implementations can be made based on what isdisclosed.

What is claimed is:
 1. A laser comprising: a layer having first andsecond regions, the first region including a void; a mirror sectionprovided on the layer, the mirror section comprising: a waveguide core,at least part of the waveguide core is provided over at least a portionof the void; a first grating provided on the waveguide core; a firstcladding provided between the layer and the waveguide core and supportedby the second region of the layer; a second cladding provided on thewaveguide core; and a heat source configured to change a temperature ofat least one of the waveguide core and the grating, wherein an opticalmode propagating in the waveguide core of the mirror section does notincur substantial loss due to interaction with portions of the mirrorsection above and below the waveguide core.
 2. The laser in accordancewith claim 1, wherein a loss incurred by the optical mode during saidpropagation in the mirror is less than 7 dB/cm.
 3. The laser inaccordance with claim 2, wherein a loss incurred by the optical modeduring said propagation in the mirror is less than 5 dB/cm.
 4. The laserin accordance with claim 3, wherein a loss incurred by the optical modeduring said propagation in the mirror is less than 2.5 dB/cm.
 5. Thelaser in accordance with claim 1, wherein the lack of substantial lossis achieved by not having a deleterious device layer in the layer.
 6. Asemiconductor device, comprising: a substrate; a layer formed on thesubstrate, the layer having first and second regions, the first regionof the layer including one or more voids; a gain section provided on thelayer, the gain section comprising: a first waveguide core; a p-njunction; and a contact layer provided on the first waveguide core, thecontact layer being configured to provide an electrical contact to thep-n junction of the gain section; a first mirror section provided on thelayer, the first mirror section comprising: a second waveguide core,wherein at least part of the second waveguide core is provided over afirst void; a first grating provided on the second waveguide core; and afirst heat source configured to change a temperature of the secondwaveguide core and the first grating; a phase section provided on thelayer, the phase section comprising: a third waveguide core; and asecond heat source configured to change a temperature of the thirdwaveguide core; a second mirror section, the second mirror sectioncomprising: a fourth waveguide core, wherein at least part of the fourthwaveguide core is provided over a second void; a second grating providedon the fourth waveguide core; and a third heat source configured tochange a temperature of the fourth waveguide core and the secondgrating; first, second, third, and fourth claddings surrounding thefirst, second, third, and fourth waveguide cores, respectively, whereinthe first waveguide core, the second waveguide core, the third waveguidecore, and the fourth waveguide cores are configured to opticallycommunicate with one another, and wherein an optical mode propagating inthe first and second mirror sections does not incur substantial loss dueto interaction with portions of the first mirror section and the secondmirror section outside the second and fourth waveguide cores.
 7. Thesemiconductor laser of claim 6, wherein the optical mode propagating inthe first mirror section or the second mirror section incurs a loss thatis less than 7 dB/cm.
 8. The semiconductor laser of claim 7, wherein theoptical mode propagating in the first mirror section or the secondmirror section incurs a loss that is less than 5 dB/cm.
 9. Thesemiconductor laser of claim 8, wherein the optical modal propagating inthe first mirror section or the second mirror section incurs a loss thatis less than 2.5 dB/cm.
 10. The semiconductor device of claim 6, whereinone or more deleterious device layers do not extend into the mirrorsections where the first void and the second void are located.